1. Field of the Invention
The present invention relates to a ferroelectric memory capable of non-destructive read-out, wherein a ferroelectric material is employed as information recording medium.
2. Description of the Related Art
In general, a ferroelectric material has a hysteresis characteristic and is employed in a ferroelectric memory device, etc. for storing data by utilizing the hysteresis characteristic.
FIG. 10A is a graph showing a P-V curve of a hysteresis characteristic of a general ferroelectric material, in which the abscissa indicates a voltage V and the ordinate a polarization P. FIG. 10B is a graph showing a C-V curve (obtained by differentiating the P-V curve with the voltage V), in which the abscissa indicates a voltage V and the ordinate a capacitance C. In these drawings, "+VC" and "-VC" indicate a voltage (hereinafter called "coercive voltage") by which directions of polarization are reversed.
As shown in FIG. 10A, the hysteresis characteristic curve of a general ferroelectric material includes one nonlinear portion in a steep rising region A (V&gt;0) and one non-linear portion in a steep descending region B (V&lt;0).
In FIG. 10A, when a voltage V is "0", there are two states of polarization, and the two states are made to correspond to "1" and "0" of digital signals. When these states are read out, a voltage higher than that of the resistant electric field is applied to a selected memory cell in a specific direction. When the direction of this voltage application coincides with the direction of voltage application at the time of write, only a slight displacement current flows. By contrast, if the direction of voltage application differs from that at the time of write, polarization reverse occurs and a large current flows. On the basis of different current values, "1" and "0" are discriminated. In a conventional destructive read-out technique, the hysteresis characteristic shown in FIG. 10 is employed.
There are generally two device structures of the ferroelectric memory utilizing the hysteresis characteristic. One structure is a simple matrix structure, wherein an intersection of stripe electrodes situated at right angles with each other on both surfaces of a ferroelectric thin film is employed as one memory cell. This structure is simple and suitable for high density. The other structure is an active matrix structure wherein one ferroelectric cell is provided with one switch element. This structure is complex and not suitable for high density.
The method of reading out data from these ferroelectric memories is a destructive read-out method utilizing a polarization reverse current, this method requiring rewrite of data in a selected cell.
These ferroelectric memories are disclosed in U.S. Ser. No. 373,082, now U.S. Pat. No. 5,060,191. This document teaches a ferroelectric memory of a simple matrix structure wherein data write/read is performed while the influence of self-reverse phenomenon a phenomenon in which the initial polarization state is restored upon application of external pulses of a ferroelectric thin film itself on a non-selected cell is suppressed by low-impedance write and read.
The above-described ferroelectric memory device, however, has the following problems.
First, when data in a selected memory cell is read out, the data of the selected cell itself is destroyed since a read-out voltage is higher than a coercive voltage. In order to retain the data, rewrite must be performed after read-out, and a complex circuit is required to rewrite data.
Secondly, according to the destructive read-out method, polarization reverse is repeated each time data write/read is carried out. Thus, ferroelectricity is degraded and residual polarization is decreased. That is, there is a problem of fatigue in increasing the life of the memory.
Thirdly, there is a problem of crosstalk between a plurality of memory cells arranged in a matrix so as to increase integration density and memory capacity of non-volatile memories. That is, when a voltage higher than a coercive voltage is applied to a selected memory cell, a voltage exceeding a coercive voltage due to crosstalk may also be applied to an adjacent memory cell and destroy the data in this memory.
Lastly, regarding the write/read method for a simple matrix memory disclosed in the aforementioned U.S. Ser. No. 373,082 (now U.S. Pat. No. 5,060,191) a specific mechanism and device structure in relation to spontaneous reverse phenomenon of spontaneous polarization have not been completed in practical stages.